I am preparing my first Japenese Curry (cheating incredibly by using the S&B Golden Curry Medium Hot box) and it suggests adding "vegetables" without actually indicating what veggies to add other than onions.

Assuming access to acceptable supermarkets (Asian and normal Australian) what vegetables (normal and/or exotic) make the best compliment to a curry like this?

The most common ones simmered along with curry are onions, carrots, and potatoes. You could even add some sliced apples. Root vegetables are frequently simmered with the curry, and you could consider using variants like sweet potato or squash, kabu, daikon, etc.

As a "topping", the sky is the limit; I've seen blanched okra, cooked renkon, roasted or simmered slices of kabocha squash, green beans, sweet potatoes, and cauliflower.